1. Field of the Invention
This invention relates to a three-dimensional image display device for displaying a still or a motion solid image with use of a hologram.
2. Description of the Related Art
Mechanical CADs usually use a three-side figure (e.g., third angle projection) or two-dimensional projection to display the shape of a three-dimensional object, etc. It is, however, rather difficult to recognize a complicated shape accurately from a two-dimensional projected image. In three-dimensional computer graphics, in order to observe an object in various directions, an image must be rotated on a display screen and hence it is difficult to grasp the whole image spatially.
A three-dimensional solid image is more preferable than a two-dimensional image to visualize a result of simulation of fluid or to display shapes of bones and blood vessels on the basis of an ultrasonic diagnosis image or a CT (Computed Tomography) image. The formation of a motion solid image is desired to obtain an ambience even in the field of solid picture images.
Conventionally, a stereo-pair display process has been used usually to display a three-dimensional solid image. The stereo-pair display process is to provide separate images for viewer's right and left eyes to obtain a depth. When the viewer observes two objects different in depth with the both eyes, the positions of the two object images formed on the retinas of his right and left eyes deviate in proportion to the difference in distance between the objects. This quantity of deviation between the object images seen by the both eyes is called a binocular parallax. By displaying two motion or still images containing information of the binocular parallax and using a pair of special purpose glasses which utilizes a difference in color or polarization and a special display screen including an array of micro optical elements, only image components corresponding to the right and left eyes can be separately observed, and a solid space can be reproduced in front and rear directions of the screen.
The stereo-pair display process, however, reproduces a space on the basis of the binocular parallax created when observed from one direction. Thus, there are the following problems:
1) Due to lack of information from other directions, variations of the image in different observation positions are not seen; PA0 2) Focus adjustment cannot be made on an object desired to be seen from the viewer; PA0 3) Since the position of the image reconstruction differs from the screen, the focus adjustment becomes a process which is not encountered in daily life; and PA0 4) Since the images viewed with the right and left eyes are different, a sensation of incongruity is felt.
A holographic display has recently been marked as means for solving such problems.
The holography is a technique for storing and reproducing both the amplitude and phase of an optical wave, so that even when a view point is moved in up-and-down or right-and-left directions, a three-dimensional image from different angles can be seen. Since the binocular parallax, jamming, and eye adjustment which are all physiological factors with which a person recognizes a solid hold, the three-dimensional image becomes more natural than that obtained by the other methods. A hologram is produced by allowing optical wave called object light from an object to interfere with different optical wave called reference light coming from a different direction and storing interference fringes occurred as a result of interference. When the reference light enters into the hologram, the reference light is diffracted by the interference fringes in the hologram resulting in formation of the same wave front as the original object light, whereby an object image appears in the space.
However, the conventional holographic display requires one hologram for one subject (one image). Thus, in the case of a solid or a stereoscopic image of one image (still image), even though its stereoscopicness is very high, a large amount of holograms are required and the device structure becomes very complicated when a motion image is displayed.
The conventional hologram has the problem that an observable visual field angle is narrow and thus the number of people who can observe at the same time is limited to less than the several persons.